Matthew Dunn, a former field officer with MI6, has lifted the lid on the Secret Intelligence Service to create a fictional spy.

There is a scar on Matthew Dunn’s right hand, a deep gash running down the edge from the little finger, as if at some point he has had to defend himself from the strike of a blade. A memento, perhaps, of the clandestine operations during his career as a field officer at the sharp end of the Secret Intelligence Service, MI6. Has he ever come close to losing his life in the service of Queen and country? His fingers drum the table.

“Can we pause there for a second?” Tap, tap, tap.

“The answer is, yes.” Tap, tap.

“Let me think how to answer that ----.” Pause.

“We’ll leave it as Yes.”

We are sitting in his new home in the south of England, hidden in autumnal woodland at the end of a long and winding drive. The house is virtually empty of furniture, the kind of place in which George Smiley might conduct one of his gently menacing interrogations. So, about that scar?

“It was earned during an operation that almost went wrong but didn’t.” A knife wound? “Well, it was sharper than a gun.” Not a DIY injury then? “I would have done a better job of stitching it up if it had been.”

Dunn is 42 and our newest spy writer. Like le Carré and Fleming before him he is the real thing, a former member of SIS turned fictional chronicler of the secret world. His, though, is a more muscular creation than Smiley, or even Bond. Meet Will Cochrane, a one-man weapon of mass destruction; 007 is a cocktail-sipping lush compared with Cochrane, a man who treats bullets in the stomach as others would an attack of dandruff.

“My experience does help enormously,” says Dunn of his new career. “Obviously, I know all the technicalities of espionage. I chose to write about a field operative because that was my speciality. As a field officer you are on your own, you make your own decisions. What is authentic in the book? Certain trade craft techniques, the isolation, the decision making. Of course, I have intensified it in the book.”

You can say that again. Dunn’s first book is called Spartan, Cochrane’s codename, and its opening pages are littered with the bodies of Iranian (and British) heavies gunned down during a spat in New York’s Central Park. Cochrane takes a few slugs but is soon up and about, on the tail of an Iranian master-spy bent on Armageddon.

Six feet four inches tall, Dunn was involved in some 70 clandestine operations, receiving a personal commendation from the then foreign secretary, Robin Cook, for his role in one episode. “I was told a commendation is very rare,” he says. “It’s upstairs but you can’t see it because it has Top Secret written on the top of it.”

What can he say of the circumstances? “Very little, beyond that it was a major incident. It was outside British territory and resolved to the satisfaction of HMG.”

Dunn was born in London, the son of a merchant seaman turned photographer. Educated at a state school, he read politics and economics at the University of East Anglia before taking a PhD in international relations at Cambridge. It was there that a tutor raised the matter of his future.

“There was a discussion about doing something academic. Part of me was attracted to that but I also had fire in the belly. I mentioned the diplomatic service and at some point he said there was another aspect of the service that might be of interest, also involving travel and complex situations. We knew what we were talking about.”

Six months of selection followed, involving tests of mental agility, role playing, repeated interviews and intrusive vetting. Five of the dozen people on his course were women, the successful ones a cross-section of society.

“MI6 is not some elitist club. The officer has to able to interact with any kind of person. You can’t have people who have lived only in a gentlemen’s club environment. It’s not like joining the Household Cavalry.”

What bound the new recruits? “We all felt anything was possible. You don’t enter with the mindset that 'this is impossible’. You just have to think of a way to do it. One of the questions in interview was, 'Who do you look up to?’ My response was, 'Frankly, nobody’. A degree of self-confidence is required, but not arrogance.”

Part of the selection process was conducted at Fort Monckton, the MI6 training establishment near Portsmouth. Candidates were sent into the naval town to test their ability to extract information.

“They might ask you to come back with 10 passport numbers,” says Dunn. “I had to sit in the pub and ingratiate myself with a group of strangers, getting as many personal details as I could. I did OK – some did tarot card readings or magic tricks to get details.”

A black tie dinner at Monckton marked Dunn’s formal acceptance into SIS. It was 1995 and he was 27. He was to find himself at the sharp end of SIS operations. “The Directorate of Requirements and Production is the operational side of the service. Officers operating in particularly hazardous areas were those who had excelled in the paramilitary side of training. I worked with the Increment, the SAS unit attached to SIS.

They provided back-up for anything that might be extremely dangerous. I was trained in close quarter and unarmed combat. I was good at it, I enjoyed it. Yes, there is an element of James Bond. There were no watches that could burn holes through walls but you might have a letter with explosives in it that could blow off a door.”

Dunn’s first job was with a team targeting rogue states. Officers are eased into operations – non-lethal ones first – often shadowing experienced colleagues. Dunn became an agent runner, responsible for about 20 in all. Some of those he recruited would have been tortured and executed if exposed.

“As an intelligence officer, you are there to listen to an agent, to get them to reveal their secrets, and then quietly to leave a country and get back to London without all the guns and glory. A thing mostly ignored in popular fiction is the terribly close bond between an officer and his agents. I used to think of my agents as a family.”

Dunn travelled under 12 separate aliases at one time, usually alone, usually reliant on his own resources. “It’s best not to bluff being a different nationality. You can sustain an accent for just over an hour, but after four hours it goes. You get briefings from people who do for a living what you are pretending to be. You may decide that this person is a good role model, so you adopt that accent, that persona.

“The loneliness, the isolation, is the real thing. The moment you step on the plane you are on your own. You are dealing with complex individuals with their own agendas. You are constantly asking, 'Is this a set-up?”

As a thriller, Spartan has pace and style, if occasionally overburdened dialogue. Abu Dhabi is unlikely to be impressed by it. Lana, the romantic interest, describes to Cochrane how the emirate’s security people beat her before extracting her back teeth and toenails with pliers. Dunn says the war on terror has posed fresh ethical challenges for his former service.

“The issue of collaborating with people who may use torture has made the work of MI6 officers very difficult. The service maintains that torture largely doesn’t work because the person tells you what you want to hear.”

Have SIS officers found themselves in dark foreign places, within screaming distance of interrogations? “I cannot imagine that arising. If an officer was in that situation I would expect him to walk out and report it. We don’t use torture and we must maintain those morals overseas. Of course, it is very hard for the field officer. There are situations in which your morality is tested to the limit, yet you have to walk away with dignity.”

Dunn quit MI6 in 2001 to raise a family. He was tempted to return following 9/11 but needed to make money – his spy’s salary being £35,000, hardly a fortune in London. Life out of the shadows, working for a recruitment company in the City of London, was a shock.

“I had done things and seen things that few have done and seen, and met people you would never believe, but to the people in the City it was, 'That’s all fine but can you make money?’ It’s moving from the Rolls-Royce of British government to a barrow boy existence.”

He divorced three years ago and cares for two young children. He has no close friends. “It’s taken me about 10 years to adjust to leaving. It’s not like a soldier coming back from the front. SIS is still around you, but you are no longer in it. You go to a restaurant and there are certain habits: checking the entrances and exits, the table spacing. I still think that way, sometimes not through paranoia, just because you do. Then I find I don’t need to be like that.”

The novel is printed in America under the title ‘Spycatcher’ published by William Morrow, an imprint of Harper Collins.

'Spartan' by Matthew Dunn (Orion) is available from Telegraph Books for £11.99 plus £1.25 p&p, to order your copy call 0844 871 1515 or go to books.telegraph.co.uk